DNA may be the most famous molecule in the world today, but it came to the attention of scientists
rather late in the history of biology. Gregor Mendel found some of the underlying regularities of heredity
almost a century before DNA was discovered. At the turn of the century scientists discovered similar
principles then rediscovered Mendel's work and rapidly realized that life was somehow encoded
in genes. Just what those genes were made
of was a mystery, but that did not prevent scientists from starting to work out the dynamics of genes
and mutations, and how new forms of
life could result from natural
selection. The Modern Synthesis of evolution, the foundation on which most research on evolution
has rested for the past 50 years, was already set in place years before DNA was discovered.

The structure of DNA
But there's no denying that the discovery of DNA was a tremendous milestone in the exploration
of evolution. While evolutionary biologists were fashioning the Modern Synthesis, geneticists around the
world searched furiously for the molecules that carried genetic information. They knew that cells contained
several different types of molecules, such as proteins and nucleic acids. But which had the capacity to
bear information and be copied into new cells? Experiments showed that nucleic acids could affect hereditary
traits. A young American geneticist named James Watson (left) was one of the researchers who
realized that the only way to determine whether they did in fact carry genes was to understand their structure.

This was an agonizing task because scientists could only see molecules
by shining x-ray beams on them, which then bounce off the atoms and strike a piece of film in various
distinctive patterns. At Cambridge University he joined up with Francis Crick (right) to
analyze the x-ray data collected by Rosalind Franklin and others. In a sudden burst of insight, Watson
and Crick built a model out of brass plates and clamps and other bits of laboratory equipment in 1953.
As they worked, they realized that nucleic acids are arranged on a twisted ladder, with two runners made
of phosphates and sugars, and a series of rungs made of pairs of organic compounds known
as bases. Years later, they won the Nobel
Prize for this frenzy of discovery of DNA's double helix.

Life's cookbook
In the years that followed, Watson, Crick, and other researchers
figured out the basics of how DNA works. Each gene, they realized, consists of a stretch of base pairs.
A single-stranded copy of the gene was created (known as messenger RNA) and transported
to protein-building factories in the
cell called ribosomes. There, the sequence of the bases guided the assembly of a string
of amino acids that became a new protein.
When a cell divides, the double helix is unzipped and the DNA is replicated. It is lifes cookbook.

Using DNA
Evolutionary biology was revolutionized by the discovery of DNA. Mutations, researchers realized,
change the spelling of the cookbook. A single base pair may change, or a set of genes may be duplicated.
Those mutations that confer a selective advantage to an individual become more common over time, and
ultimately these mutant genes may drive the older versions out of existence.

Thanks to the discovery of DNA, it is now possible for scientists to identify not just the genes,
but the individual bases. Before the discovery of DNA, scientists could only uncover the evolutionary
tree of life by comparing the bodies and cells of different species. Now they can compare their genetic
codes, working their way down to the deepest branches of life dating back billions of years.